When I replaced my clutch, last summer, the carbon throwout bearing still had quite a bit of material left. It could have gone another 10,000 miles or so. I've heard this from others as well; the carbon bearing vitually always outlasts the rest of the assembly. So why go to the added expense of a roller throwout bearing?
No one in his right mind would even think about replacing a clutch without replacing the throwout bearing, so you won't save any money by potentially increasing the life of the bearing.
I have heard of a very few cases of the carbon ring in the bearing breaking and pieces falling out, but that seems to be very rare. I've also heard from several people that the roller bearings sometimes don't last as long as the clutch.
The key to good throwout bearing life is to never use the clutch unless you're changing gears. That means starting the car in neutral, with the clutch pedal released and putting the car into neutral at stop lights. Never sit at a light with your left foot holding the clutch pedal down and never, ever, use the clutch to hold the car on a hill.
